In recent years, research has been proceeding on implementation of Intelligent Transportation Systems (ITS) in order to improve safety, efficiency, and comfort of road traffic. ITS enables vehicles on the road or pedestrians to share various kinds of information by using, for example, Dedicated Short-Range Communications (DSRC). In DSRC, information is exchanged, for example, between a communication unit mounted on a vehicle and a roadside unit installed along a road or between vehicle-mounted communication units.
In DSRC, when a vehicle equipped with a communication unit enters the communication range of a roadside unit, a communication channel is established between the vehicle-mounted communication unit and the road side unit so that they can exchange information with each other. To enable this, a technology has been developed that assigns a communication channel between a vehicle-mounted communication unit and a roadside unit (refer, for example, to Japanese Laid-Open Patent Publication Nos. H06-350503 and 2000-358265).
In one example of such prior known art, mutually different specific pseudo-random noise codes in code division multiple access are assigned to a pilot channel, an uplink control channel, and a downlink control channel, respectively. Then, the roadside unit transmits data on the downlink control channel and voice channel in synchronism with the pilot channel. On the other hand, the mobile unit performs the despreading and demodulation of the downlink control channel and voice channel based on the timing of pilot channel synchronization seizure, and performs processing to detect distance from the correlation output of the pilot channel synchronization seizure. The mobile unit changes the phase of the pilot channel between adjacent roadside units. In another example of the prior known art, a time slot is assigned to each entering vehicle. In this case, the vehicle is assigned a time slot that is vacant not only in the current radio zone but also in the radio zone located adjacent to the current radio zone in the traveling direction of the vehicle.